To operate in most controlled airspace within the United States today, a transponder, which responds to interrogation from surveillance radar systems, is required. In response to an interrogation, a transponder transmits altitude information (Mode C) and a four digit identifying “squawk” code to the radar site (Modes A and C). The squawk code to be transmitted is entered in the cockpit. When flying visual flight rules (VFR) in Class E airspace in the United States, the code 1200 is typically squawked. When entering congested airspace, such as Class B, C, or D airports, when flying in Class A airspace, or when flying under instrument flight rules (IFR), air traffic control (ATC) may assign and request an aircraft to squawk a particular identifying code.
Presently, next generation technology, in the form of ADS-B, is being implemented in the United States. ADS-B is an aircraft surveillance and traffic management technology and system for enhancing situational awareness, in which location messages are sent periodically by aircraft without the need for interrogation from a ground station. The system is dependent on aircraft being equipped with high integrity position sources, such as Wide Area Augmentation System (WAAS) Global Positioning System (GPS) receivers. ADS-B provides real-time surveillance services to both Air Traffic Control stations and to appropriately equipped aircraft.
ADS-B makes use of two operating frequencies—978 MHz and 1090 MHz: Aircraft may transmit position information (referred to as ADS-B OUT) at 1090 MHz using an Extended Squitter equipped transponder or, if limited to operating below flight level 180 (approximately 18,000 feet), at 978 MHz using Universal Access Transceiver (UAT). ADS-B OUT information may be directly received by other similarly-equipped aircraft and by ground stations within line-of-sight.
Aircraft may receive position information of other aircraft and about obstacles, as well as weather and other information (referred to as ADS-B IN). ADS-B equipped 1090 MHz aircraft may directly receive both position information from other ADS-B equipped 1090 MHz aircraft and Traffic Information Services Broadcasts (TIS-B) from ground stations. Similarly, ADS-B equipped 978 MHz aircraft may directly receive both position information from other ADS-B equipped 978 MHz aircraft and TIS-B from ground stations. TIS-B provides traffic and obstacle information within a cylindrical volume of airspace about the aircraft. TIS-B data includes self-reported position data from both 1090 MHz and 978 MHz ADS-B OUT equipped aircraft and basic position data from non-ADS-B OUT equipped aircraft within radar range of the ground station. Moreover, ADS-B equipped 978 MHz aircraft may receive Flight information Services Broadcasts (FIS-B) which includes subscription-free graphical and textual weather data. Due to congestion of the 1090 MHz frequency, FIS-B is only provided at 978 MHz.